Full penetration laser welding of thick steel plates has been carried out using a 20 kW CO2 laser facility. Dynamic keyhole behaviour has been observed using an x-ray transmission imaging system to elucidate the formation mechanism of the porosity and hot cracking for various welding conditions. Relatively wide optimum power range to prevent the porosity can be attained when the back surface is shielded by the inert gas. However, the convex shaped weld cross-section, which is susceptible to hot cracking, is formed due to significant perturbation of the keyhole caused by the back shielding gas flow. The keyhole is stabilised without using the back shielding. However, large porosities are likely to occur near the bottom surface, since the plasma formed just below the keyhole supplies monatomic nitrogen, which can easily dissolves into the molten pool. Supersaturated nitrogen forms bubbles in the molten pool during welding and causes the porosities. A small amount of aluminium addition in the base metal is effective to prevent this type of porosity due to denitrification effect of aluminium.

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